In recent years, in an access network for providing a multimedia service to homes, a point-to-multi-point access optical communication system called PON (Passive Optical Network) system realized by a public circuit network using an optical fiber is widely used.
The PON system is configured from one OLT (Optical Line Terminal), which is an optical termination device of a station-side apparatus, and ONUs (Optical Network Units), which are a plurality of subscriber-side terminal devices, connected via an optical star coupler.
Light reception levels of optical signals received by the OLT from the ONUs depend on the distances between the ONUs, which are transmission sources of the optical signals, and the OLT. However, the distances between the OLT and the ONUS are not the same concerning all of the ONUs. Therefore, an optical receiver used in the OLT is required to have a wide dynamic range characteristic for stably reproducing packets having different light reception levels. To realize the wide dynamic range characteristic, in general, a preamplifier mounted on the optical receiver includes an AGC (Automatic Gain Control) circuit.
In the PON system, while a certain GNU is transmitting a packet, the other ONUs cannot transmit packets. To improve transmission efficiency, time among the packets needs to be reduced. A specific bit called preamble is stored in the head of the packet transmitted from the ONU. The preamble is used for synchronization. To improve the transmission efficiency, a short preamble has to be used to receive the following payload in synchronization with the preamble. Therefore, the AGC circuit included in the preamplifier is required to have a high-speed burst reception characteristic for completing AGC convergence at high speed using the short preamble.
However, the optical receiver including the preamplifier is required to have, simultaneously with the high-speed burst reception characteristic, high identical code succession tolerance capable of stably performing reception even during identical code succession bit input. In general, the high-speed burst reception characteristic and the identical code succession tolerance are in a tradeoff relation. It is difficult to achieve both of the high-speed burst reception characteristic and the identical code succession tolerance.
As the optical receiver of the OLT, some optical receiver includes an AOC (Auto-Offset Control: auto-offset voltage adjustment or auto-offset compensation) circuit that compensates for an offset voltage between input burst signals or differential output signals of an internal differential amplification circuit. Like the AGC circuit, the ACC circuit is required to have both of the high-speed burst reception characteristic and the identical code succession tolerance. That is, voltage control circuits such as the AOC circuit and the AGC circuit in the optical receiver of the OLT are required to have both of the high-speed burst reception characteristic and the identical code succession tolerance.
Therefore, Patent Literature 1 and Patent Literature 2 disclose control schemes for switching a tame constant. In Patent Literature 1 described below, an amplification circuit includes an AOC circuit that compensates for an offset voltage of an input burst signal and outputs the offset voltage on the basis of an offset voltage detected and retained at a time constant variably controlled by a time constant control signal, a pulse detection circuit that detects presence or absence of a pulse from the burst signal and outputs a pulse detection signal; and a time-constant control circuit that outputs, on the basis of the pulse detection signal, to the AOC circuit, a time constant control circuit for reducing, that is, shortening the time constant in a pulse detection section and increasing, that is, lengthening the time constant in a pulse non-detection section. The amplification circuit includes an LIA (Limiting Amplifier) that is connected to, for example, a post stage of a preamplifier and amplifies and limits the burst signal to have constant amplitude.
For example, when there is offset between differential signals input to the LIA, the burst signal is amplified and limited while the offset voltage is kept applied to the burst signal. Therefore, the offset voltage affects an output burst signal of the LIA in a form of waveform distortion and deteriorates waveform quality. Therefore, as described in Patent Literature 1 described below, in general, the amplification circuit includes the AOC circuit. A time constant switching scheme for achieving both of the high-speed burst reception characteristic and the identical code succession tolerance is used. The amplification circuit described in Patent Literature 1 described below detects presence or absence of a pulse and is controlled to reduce the time constant in the pulse detection section to make it possible to output an output burst signal having a stable waveform in a short time and, on the other hand, is controlled to increase the time constant in the pulse non-detection section and can suppress fluctuation in a control signal of the AOC circuit even in a section in which identical code bits are consecutively input.
In Patent Literature 2 described below, a time constant switching scheme same as the time constant switching scheme in the Patent Literature 1 described below is applied in an amplification circuit. When there is a voltage signal output from the LIA, the time constant of the AOC circuit is set to a small value to improve initial response speed of feedback compensation. On the other hand, when there is no voltage signal output from the LIA, the time constant is increased to suppress baseline fluctuation of a signal even if identical code bits are consecutively received. An eye aperture can be increased and stability is improved. In Patent Literature 1 described below, time constant control is realized by using the pulse detection circuit. In Patent Literature 2 described below, the time constant control is realized by using a signal detector (SD) that detects a voltage signal output from the LIA.